1. Field of the Invention
This invention relates to an ultraclean evaporation system and, more specifically, to such a system capable of analyzing ultrapure hydrogen peroxide down to parts per trillion or quadrillion.
2. Brief Description of the Prior Art
Impurity analysis by evaporation techniques has been known in the prior art. These techniques have operated well since extremely low detection limits (i.e., 1 part/10.sup.12 to 1 part/10.sup.15) have not been required. However, with the continual reduction in the geometry of semiconductor devices, impurities in the involved chemicals take on greater importance because they can become an increasingly larger proportion of the volume of individual components. These impurities can and often degrade the electrical performance of the components of which they are a part. It follows that impurity concentration must be more accurately monitored.
Prior art evaporation techniques fail to provide accuracy levels of the type required for many of the newer generations of semiconductor devices due to contamination of the sample during evaporation, poor recoveries at the part per trillion to part per quadrillion level, slow evaporation times, uneven heating and poor removal of evaporated liquid from the system.
An additional problem is present during the evaporation of hydrogen peroxide for purposes of impurity measurement in that the risk of an explosion increases with the increase in the percent of hydrogen peroxide in the vapor phase. As evaporation continues, generally the water portion of the liquid phase evaporates first with the concentration of liquid phase hydrogen peroxide increasing with decrease of the volume of the liquid phase. As the percentage of water in the liquid phase decreases, the vapors will increasingly contain hydrogen peroxide vapor. Hydrogen peroxide vapors greater than 26 mole percent at atmospheric pressure are extremely explosive and accordingly present the hazard.
Furthermore, traditional evaporation schemes are plagued with slow evaporation times, uneven heating, environmental contamination and poor analyte recoveries at the part per trillion and part per quadrillion levels.